Greenhouse gases

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Greenhouse effect

The atmosphere consists of about 78 per cent nitrogen, 21 per cent oxygen and a range of other gases including water vapour and CARBON DIOXIDE. Carbon dioxide currently makes up about 0.038 per cent of the atmosphere, and water vapour varies from a fraction of 1 per cent to about 3 per cent.

Greenhouse gases are gases in the atmosphere that absorb and emit infrared or heat radiation, thus trapping heat in the lower atmosphere. Most greenhouse gases exist in the atmosphere naturally at very small concentrations. The major atmospheric gases, nitrogen and oxygen, which together make up 99 per cent of the atmosphere, are not greenhouse gases.

Several gases together cause the natural greenhouse effect. One of the most significant is ordinary water vapour. Carbon dioxide, which is a vital natural component of the atmosphere, is also responsible for some of the heat-trapping of the natural greenhouse effect. Without these small quantities of naturally occurring greenhouse gases, our planet's average temperature would be more than 30°C colder than it actually is.

In the last century or so, industrial and agricultural activities have caused an increase in the quantity of most of these natural greenhouse gases (apart from water vapour). In particular, the burning of carbon-containing fuels (such as oil products, coal and methane) has increased the concentration of carbon dioxide by more than one third. This increase in greenhouse gas concentrations is increasing the natural greenhouse effect, and is considered to be responsible for most of the measured increase in global temperature that has occurred over the last century. This is known as the anthropogenic (human-caused) greenhouse effect, or the enhanced greenhouse effect.

Water vapour:

The main natural greenhouse gas is water vapour. Water vapour is always present throughout the lower atmosphere, even if sometimes at a very low leve. Water is constantly transferred between the oceans, atmosphere and land in the global hydrological cycle, or the water cycle. When condensed as liquid or ice droplets, water is the main constituent of clouds.

Although human activities affect the water cycle, they do not appear to have directly changed the concentration of water vapour globally. As will become clear below, water vapour is therefore not measured as part of anthropogenic—human-generated—greenhouse gas emissions. It is also worth noting that although water as a gas traps heat in the lower atmosphere, when it is in the form of suspended droplets (essentially clouds), it can also act to cool the surface of the earth.

Carbon dioxide (CO2):

The other major greenhouse gas, and the one most often discussed, is carbon dioxide. Carbon dioxide makes up a small but growing component of the atmosphere. Its current concentration is 0.038 per cent, or 380 parts per million (ppm). Two hundred years ago, its concentration was only about 280 ppm.

At very small concentrations, carbon dioxide is a natural and essential part of the atmosphere, and is required for the photosynthesis of all plants.

Carbon dioxide enters and leaves the atmosphere from a number of natural sources and sinks at the earth’s surface. In the absence of human influences, the fluxes of carbon dioxide into and out of the atmosphere were largely in balance,. The burning of carbon-containing fuels (coal, oil and gas) has considerably increased the concentration of the CO2 in the atmosphere. Scientists can directly measure the concentration of carbon dioxide in the atmosphere; they can also determine atmospheric carbon dioxide concentrations in the past by, for example, measuring the gas in tiny air bubbles trapped for many thousands of years in deep layers of polar ice.

Large amounts of carbon dioxide are transferred between the atmosphere, oceans and land vegetation in the natural global carbon cycle. Anthropogenic emissions are adding to the amount of CO2 in the atmosphere, and causing changes in the amount taken up by the oceans and vegetation. The main stores and fluxes of carbon in the global carbon cycle are described and illustrated in sinks and sources.

Other greenhouse gases:

There are other gases that contribute to the greenhouse effect. Many of these are produced or augmented by human activity. Two important examples are the gases METHANE and NITROUS OXIDE.

Methane was once called 'marsh gas' and is the main constituent of the gas which can cause explosions in coal mines. Methane is the principle component of the 'natural gas' that is used as a fuel (when combusted, the carbon in methane is oxidised to produce carbon dioxide as a waste product; however, there are also significant fugitive or unintentional emissions of methane involved in the extraction, transport and processing of natural gas); it is manufactured as 'biogas' from the decomposition of organic matter; and is also produced by the processes of digestion in ruminant animals, including cattle and sheep.

Nitrous oxide is a powerful greenhouse gas that is produced naturally by microbes in the soil and ocean, and is a by-product of agricultural activity involving nitrogen fertilisers and animal wastes. It is also given off in small quantities by the burning of fossil fuels including oil and coal.

CHLOROFLUOROCARBONS (CFCs), HYDROCHLOROFLUOROCARBONS (HCFCs) and other carbon compounds containing chlorine and bromine are strong greenhouse gases. Emissions of these gases have been curtailed due to regulation under the Montreal Protocol, but together they have been responsible for about 12 per cent of the enhanced greenhouse effect. Most of the contribution has been from CFCs.

Global warming potentials:

Greenhouse gases differ in their ability to trap heat, as well as in their concentrations in the air. Many of these gases have a far greater warming effect than carbon dioxide for a given mass, though their warming contribution overall is less because their concentration in the atmosphere is much lower. The length of time that their warming effect can persist also differs, as it depends on how long they remain 'resident' in the atmosphere before natural chemical or physical processes remove them.

To be able to compare the warming effects of the different greenhouse gases, scientists have calculated the 'global warming potential' (GWP) of each gas. The global warming potential measures how much a particular greenhouse gas contributes to global warming. The GWP compares the RADIATIVE FORCING, or warming ability, of a particular gas to that of carbon dioxide, which is used as a reference. The warming potential of carbon dioxide is given a value of one, against which the other gases are compared.

As explained, each gas remains for a different period of time in the atmosphere before it is removed by natural processes. Hence the global warming potential for each greenhouse gas is calculated over a specific time interval; standard time intervals for these calculations are 20 years, 100 years and 500 years. It is appropriate to quote the length of time whenever a global warming potential figure is quoted. The standard used to calculate carbon dioxide equivalents (see below) is 100 years. For example, the greenhouse gas NITROUS OXIDE has a global warming potential of 298. This means that one tonne of nitrous oxide in the air has the same effect as 298 tonnes of carbon dioxide over a 100-year time frame.

The global warming potential of the major greenhouse gases over a 100-year time horizon are shown in the table below.

Note that some of these gases have a very large global warming potential. This means that extremely small quantities can have large effects in comparison to carbon dioxide. HYDROFLUOROCARBONS (HFCs) are used in refrigeration and air conditioning; they are used as replacements for CFCs—CHLOROFLUOROCARBONS—which are ozone-depleting gases. PERFLUOROCARBONS were introduced as alternatives to the ozone-depleting HFCs but have relatively high global warming potentials. HYDROFLUOROETHERS are more recent replacements for CFCs and HFCs; they have lower ozone-depleting properties and generally lower global warming potentials compared with HFCs and PFCs over a 100-year time scale, because they tend to have a shorter lifetime in the atmosphere.

Carbon dioxide equivalents:

Much technical literature about the enhanced greenhouse effect refers to carbon dioxide equivalents. This is a way of making it easier to lump together the warming effects of several different gases. A known mass of another greenhouse gas is translated to an equivalent mass of carbon dioxide, by using its GWP. For example, in 'greenhouse accounting', one tonne of nitrous oxide, which has a GWP of 298, will be counted as 298 tonnes of carbon dioxide equivalents (called CO2-e).